The present invention relates in general to steam generators, and in particular to a new and useful method and apparatus for controlling the output temperature of a superheater in a steam generator.
The normal method of controlling the superheater outlet temperature from a steam generator is by the use of a water attemperator located either at the superheater outlet or, more commonly, between the superheater stages, i.e. at the outlet of the primary superheater and before the inlet of the secondary superheater. This control system is normally designed to provide a feed-forward of the spray demand to an attemperator control valve, to improve control stability. The feed-forward control uses unit load and secondary superheater inlet steam temperature as an index. The limited variation of this steam temperature with load does not provide the feed-forward control of spray flow needed for a dynamic system, however.
U.S. Pat. No. 4,289,114 discloses a control system for a solar powered steam generator which controls its attemperator control valve using the mid-range of a signal which is formed as a function of the secondary superheater outlet temperature, process set points, and the attemperator temperature. The high and low range of the same signal is utilized to form a total feed-forward demand for the boiler feed water, as a function of total steam flow from the solar steam generator. The total steam flow is calculated from the sum of turbine steam flow as measured by the first stage pressure in the turbine and steam flow to storage, less steam flow from storage to the turbine.
U.S. Pat. No. 4,776,301 discloses a control system for generating a feed-forward signal which can be used to control a spray attemperator, the feed-forward signal including a computed value for heat absorption in the superheater required to maintain an enthalpy of the steam discharge from the superheater at a set point value. Known functional relationships exists between the enthalpy of steam, and its pressure and temperature.
U.S. Pat. No. 3,894,396 provides an overview of a typical steam generator operation including a furnace with economizer, primary and secondary superheaters and reheater, as it is used to generate steam to drive high, intermediate and low pressure turbines which in turn, drive a generator for generating electricity. This patent also discloses the use of separate control loops, each operating in parallel, including an auxiliary control for a spray type attemperator.